111Dr Omar Abdel Aziz, vice-chair at ASHRAE Falcon Chapter, has discussed the role of the CO2 refrigeration system to achieve energy efficiency targets

Natural refrigerants, being at the forefront of sustainable freezing and chilling solutions, have been used since the start of the chilling technology. Ammonia and carbon dioxide (CO2) were among the first to be used in the early days of refrigeration in the 19th century. Now ammonia, CO2 and hydrocarbons continue to be the natural refrigerates of choice for freezing and chilling applications.

CO2 in energy-efficiency

CO2 as a refrigerant is a very good heat transfer fluid, which significantly reduces the heat transfer area and improves its performance compared with other refrigerants. However, one issue with CO2 refrigerant is that it has low critical temperature, and as such can only operate at mild ambient conditions or in air conditioned spaces; its performance degrades significantly in hot environment due to the transcritical operation.

To overcome this issue, a cascade system can be used where a secondary chilling system can be used outside the building. Here, the CO2 is used inside the store where it run throughout the store in small diameter pipe network. Another high efficiency refrigeration unit running with a reduced charge of another refrigerant is installed on the rooftop or outside the building to cool down the CO2.

In the cascade configuration, CO2 is used as the heat transfer medium inside the store. Therefore, instead of distributing the refrigerants inside the store, the operator can have CO2 throughout the store. Then the operator has another refrigeration cycle outside the store that cools the CO2. This system can be ammonia or hydrocarbon or a synthetic refrigerant like R404A.

The good thing is that the operator has more control over the design of this outside equipment. It is much smaller refrigerant charge than the convenient system. So the overall global warming potential of the system is much reduced compared to traditional refrigeration.

Economically friendly too

For the UAE, CO2 as a refrigeration system are made up of welded pipe network instead of brazing; this would significantly reduce the potential for refrigerant leakage and the need to top of the system to maintain proper performance. Secondly, CO2 systems require a smaller real estate size. They are much more compact, so the operator can have more sales volumes. In addition to this, in the CO2 system, there is a trade-off between the cost of the pipe and the high pressure, which lowers the installation cost.

Environmentally speaking, CO2 is friendly because it has global warming potential of one than compared with the conventional R22 refrigerant which has a global warming potential of 1760 and R404A which has global warming potential close to 4,000.

Next-generation natural refrigerants

Globally, there is a push for natural refrigerants because natural refrigerants are eventually thought to be crucial in keeping environmental sustainability. Ammonia is a toxic flammable refrigerant, hydrocarbons are flammable refrigerants, CO2 has some limitations as I described earlier plus it is a high-pressure refrigerant.

In the meantime, the industry has answered this problem by developing an intermediate synthetic refrigerant that is based on hydrofluoroolefins or unsaturated olefins which have very low global warming potential and zero to extremely low ozone depleting potential. Some of these hydrofluoroolefins compound include R1234ze(E) which is acceptable for refrigeration as well as R448A and R449A that are introduced for the refrigeration as replacements for the high global warming potential refrigerant R404A.

In regard to the CO2 refrigeration system, it is important that each application be well-designed. They are mostly custom-made designs, especially for the UAE’s arid climate. Also, the operator should look at the good demonstration of the technology in the Middle East and try to get this technology into the regional marketplace.

ASHRAE in the UAE

ASHRAE as a global organisation is developing a lot of standards that help countries and organisations achieve their sustainability and energy-efficiency targets. These include the ASHRAE 90.1 standard and the refrigeration commissioning guide as well as the different test standard for refrigeration equipment.

In the UAE, ASHRAE helps educate the contractor by doing a lot of activities in this regards. Most recently, the ASHRAE Falcon Chapter has translated the ASHRAE standard 90.1 and the ASHRAE standard 62.1 into Arabic. These are two of the most relevant standards in terms of energy efficiency.

Novotel Al Barsha
Dubai, UAE
February 24th - 25th
2019